A sleeping bag is designed to catch air between it’s fibers. In-turn, this air is warmed by your body and you stay warm. The area of fluffiness is called the loft of the bag. The greater the loft, the warmer the bag.

Down

Different materials capture air better than others. Down (the small feathers found closest to the skin of a duck or goose) is ideal for this purpose as it possesses many small fibres. Down is graded by number; the 800 and 900 series Down being the best/warmest.

Down is often mixed with feathers (non-down) and used as an insulator for less expensive ‘Down Sleeping Bags’. So if you purchase one, be sure to note what you are buying. Not all down bags are created equally.

Down does require to be shaken to increase it’s loft after it is crammed into a stuff sack. Some users fail to fluff-up there bag before use and wonder why they were cold…

Down is lighter than Synthetic fibres and as such, is sought after by hikers and climbers. Lightweight and pound for pound the warmest material you can buy.

Synthetic

Synthetic fibres are man made. Although they can be very good (almost as warm as down) they are plastic based. As I’ve already discussed, they weigh more, so it seems that in comparison, there’s not much of a contest. Well that conclusion isn’t entirely correct…

Moisture: Down’s Downfall

If a Down Bag gets wet, all of its lofting properties are removed. The bag will not keep you warm.

This is not limited to the bag falling into water or getting rained on. Your body continually exhales moisture. Once inside the bag, the moisture will be absorbed by the bag. If the bag isn’t dried (air out) every morning, it will continue to absorb moisture the following night (and so on). The bags insulation qualities will continue to decrease until it doesn’t provide the adequate insulation required and you get cold.

As synthetic bags use plastic fibres, they don’t mat together in the same way as down. The bag will absorb some water, but 100% of its loft is maintained. The bag is warm even soaking wet.

My Choice

Unless I was expecting extremely cold dry conditions (with ample time to air out the bag), was pressed to be extremely weight conscious and the trip was short, I wouldn’t use down.

I too use to be a diehard down user and still have several 800 and 900 series down vests and jackets/pants that I use. But I always select a synthetic bag.